Configurational heterogeneity

Difficult samples for investigation are those which present absorbing atomic species in a variety of configurations (heterogeneity). The presence of absorbers in different coordination geometries often leads to featureless XANES. 

ZPR>6 Assembly 7 Plate>Rod Configuration Heterogeneity Measurements (ail Rieasurements at room temperature)... 

There is an obvious reluctance of carbohydrates to crystallize in a form suitable for X-ray or neutron diffraction studies. This is particularly true for aldose and ketose containing carbohydrates as a configurational mixture of four isomers, a-and P-pyranoses and a- and P-furanose, is likely to occur. Configurational heterogeneity in solution tends to inhibit crystallization. For this reason 1-O-methyl derivatives, which cannot epimerize, are likely to crystallize more readily. In some instances, the a and P epimers can co-crystallize, and more than 10 examples of such co-crystallization can be found. The a and p ratio may be dependent on the temperature and solvent of crystallization, and may not be reproducible between independent investigators. One extreme example is provide by the crystal struc-... 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

It is basically a fractionation process that depends not only on molecular size, but also on chemical composition, stereo-configuration, branching, and crosslinking. For multicomponent systems, fractionation with different ion polymolecularity, chemical heterogeneity and sequence length distribution, solubility or elution fractionation is of primary importance. Therefore, gel permeation chromatography or size exclusion chromatography is used as an important tool for the characterization of PBAs. 

Bertrand and Santelli (89) have investigated the hydrolysis, under apparently heterogeneous conditions, of 4,5-hexadien-2-yl tosylate, 114, and 4-methyl4,5-hexadien-2-yl tosylate, 115. After 90 min at 80°, 114 upon steam distillation yields a mixture consisting of 3% hydrocarbon 67% of cis- and rrans-2-methylcyclopropyl methyl ketone, 116 and 30% of 4,5-hexadien-2-ol, 117. When optically active 114 was treated under similar conditions, the resulting rrans-2-methylcyclopropyl methyl ketone had an inverted configuration at the reaction center, whereas the dienol 117 was found to be racemic (90). This... 

Another problem that required solving was the moderate yield obtained in the cyclopropanation reaction when only one equivalent of styrene was used. By increasing the amount of styrene up to its use as the reaction solvent, a noticeable effect on the selectivities was observed when laponite was used as the support [58]. The active role of the clay support was definitely estabhshed when the results in homogeneous and heterogeneous phases were compared (Table 9). These effects involved the reversal of the trans preference in solution to the cis preference with the laponite-supported catalyst in styrene, and also a reversal in the absolute configuration of the major cis enantiomer ob-... 

Knudsen diffusion is generally encountered in heterogeneous catalysis. In the smallest pores configurational diffusion takes place with diffusion coefficient ranging from 10 to 10 m /s. 

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